Many phones these days provide an inbuilt app locker. You can find this option as App Locker, generally under the Utilities option. You can simply open the option and select an app which you wish to lock. Now everytime you wish to open that app, you will have to enter the phone lock password or authenticate using fingerprint sensor, if your phone supports fingerprint.

Alternatively, if your phone does not support this feature, you can download the app named app locker from play store.